Spinal Fastener With Serrated Thread

ABSTRACT

A fastener configured for spinal applications includes a head having a channel adapted to receive a spinal rod and a shaft extending from the head to a distal tip and having a thread, at least a portion of the thread being serrated. The serrated portion of the thread includes peaks and troughs and can extend along about 35 percent of a length of the thread.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/645,264, filed on Jul. 10, 2017, which claims the benefit ofthe filing date of U.S. Provisional Patent Application No. 62/428,103filed Nov. 30, 2016, the disclosures of which are hereby incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to spinal fixation devices, andmore particularly to spinal fasteners having serrated threads.

A technique commonly referred to as spinal fixation is employed forfusing together and/or mechanically immobilizing vertebrae of the spine.Spinal fixation may also be used to alter the alignment of adjacentvertebrae relative to one another so as to change the overall alignmentof the spine. Such techniques have been used effectively to treat manydegenerative conditions and, in most cases, to relieve pain suffered bythe patient.

In some applications, a surgeon will install pedicle screws into thepedicles of adjacent vertebrae (along one or multiple levels of thespine) and thereafter connect the screws with a spinal rod in order toprovide immobilization and stabilization of the vertebral column.Whether conducted in conjunction with interbody fusion or across singleor multiple levels of the spine, the use of pedicle screws connected byfixation rods is an important treatment method employed by spinalsurgeons.

Some surgeons insert pedicle screws via powered screw insertion, whileother surgeons prefer manual screw insertion. For the surgeons that optfor manual screw insertion, surgeon fatigue and bone fracturing can besignificant problems during surgery. Surgeon fatigue can adverselyaffect the accuracy of the insertion process and the depth to which thescrews are inserted within the pedicle bone.

There remains room for improvement in the design and use of pediclescrews, particularly in the case of manual insertion so that relatedsurgical procedures can be performed with greater efficiency andconsistency.

BRIEF SUMMARY OF THE INVENTION

A first aspect of the present invention is a fastener having a headincluding a channel adapted to receive a spinal rod and a shaftextending from the head to a distal tip and including a thread, at leasta portion of the thread being serrated.

In other embodiments according to the first aspect, the shaft has alongitudinal axis and an angle between the longitudinal axis and threadmay vary along a length of the shaft. The serrated portion of the threadmay include serrations having a width that increases along a portion ofa length of the thread toward the distal tip. The shaft may becannulated. The head may be polyaxially movable with respect to theshaft. The shaft may be tapered. Further, the taper of the shaftmeasured by a line over a surface of the thread at points on two or morerevolutions of the thread may be between 16 and 20 degrees relative tothe longitudinal axis of the shaft. The serrated portion of the threadmay include serrations having a width that decreases over a part of theserrated portion toward the distal tip. The thread may include wallsdisposed between the shaft and a surface of the serrations, the wallsangled so that walls adjacent to one another along the longitudinal axisare at a 55 to 65 degree angle with respect to each other. The head maybe monoaxially attached to the shaft.

A second aspect of the invention is a fastener having a head including achannel adapted to receive a spinal rod, a shaft coupled with the head,the shaft including a distal tip, a thread extending between the headand the distal tip, and a serration extending along at least a portionof the thread, the serration including peaks and troughs.

In other embodiments according to the second aspect, the peaks at aradial distance to the longitudinal axis of the shaft greater than aradial distance to the longitudinal axis of the shaft from the troughsadjacent to the peak, the teeth may have a width measured parallel tothe troughs such that the width may be greater at the troughs than atthe peaks. The peaks of the teeth may include a first type defined by anedge at an abutment between surfaces connecting the peak with adjacenttroughs and a second type defined by a planar surface. The serration mayinclude a progressively increasing pitch from the tip toward the head.The first peak may vary in height along a length of the thread so that afirst short peak with a first radius measured from the longitudinal axisof the shaft may be adjacent to a first tall peak with a second radius,which in turn may be adjacent to a second short peak with a thirdradius, adjacent to a second tall peak with a fourth radius, the firstand third radii may be similar and may both be lesser in dimension thanthe second and fourth radii.

The peaks may extend along helical curves winding around the shaft in adirection opposite to a helical curve along which the thread extends.The peaks may extend along axes that are parallel to or aligned with alongitudinal axis of the shaft. The shaft may include a cutting flutethat extends in a linear direction along an axis angled with respect toa longitudinal axis of the shaft. The shaft may include a cutting flutethat extends along a helical path from the distal tip of the shaft.

A third aspect of the fastener is a fastener having a head including achannel adapted to receive a spinal rod, a shaft coupled to the head,the shaft including a distal tip, a thread extending between the headand the distal tip, and a serration extending along approximately 35percent of a length of the thread. In other embodiments according to thethird aspect, the serration may include peaks and troughs. The distaltip may taper such that an angle between an axis measured from a firstpoint on a surface of the thread at a first end of the taper to a secondpoint at the tip of the fastener on the longitudinal axis of the shaftmay be approximately 20 to 30 degrees relative to the longitudinal axisof the shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a fastener in accordance with a firstembodiment of the present invention.

FIG. 2 is a front plan view of the fastener of FIG. 1.

FIG. 3 is an enlarged front plan view of a distal portion of thefastener of FIG. 1.

FIG. 4 is a top plan view of the fastener of FIG. 1.

FIG. 5 is a bottom plan view of the fastener of FIG. 1.

FIG. 6 is an enlarged bottom plan view of a portion of the fastener ofFIG. 1.

FIG. 7 is a perspective view of a fastener in accordance with anotherembodiment of the present invention.

FIG. 8 is a perspective view of a fastener in accordance with anotherembodiment of the present invention.

FIGS. 9A-9Q are perspective views of distal portions of otherembodiments of fasteners in accordance with the present invention.

FIGS. 10-13 are perspective views of fastener in accordance with otherembodiments of the present invention.

FIG. 14 is a chart of mean maximum insertion torque for differentversions of fasteners in accordance with the present invention.

DETAILED DESCRIPTION

The present invention relates to a fastener to be used in conjunctionwith spinal rods during spinal surgery. Those of skill in the art willrecognize that the following description is merely illustrative of theprinciples of the invention, which may be applied in various ways toprovide many different alternative embodiments.

FIGS. 1-6 depict a first embodiment of a fastener 100 that is configuredfor spinal applications, and in particular, for use as a pedicle screwor fastener. Fastener 100 includes a screw body 101 and a tulip, whichhas a channel adapted to receive a spinal rod. A spinal rod can beinstalled into the tulip and held in place by a set screw (not shown),which is threaded into internal threads of the tulip. While the tulip isnot shown in FIGS. 1-6, tulips are featured in the embodiments shown inFIGS. 10-13, described below.

Fastener 100 is poly-axial in that screw body 101 is separate from thetulip. The tulip and proximal end of the screw body can generally bereferred to as a head of fastener 100. Screw body 101 includes a shaft103 that extends along a longitudinal axis 108 from a proximal portion102 or a head of fastener 100 to a distal tip 105. The tulip ispolyaxially movable (i.e., a polyaxial pedicle screw) with respect toproximal portion 102 of screw body 101. Proximal end 102 of screw body101 forms an interference fit connection with a distal opening of thetulip to create the poly-axial connection. The tulip can swivel aboutand form different angles with screw body 101 to facilitate proper rodplacement. In other embodiments, the fastener can be a monolithicstructure (i.e., a monoaxial pedicle screw) having the tulip staticallyconnected with the proximal end of the screw body. Both of suchembodiments may additionally have retractor blades extending from thetulip, such as those described below in connection with FIG. 12.

Shaft 103 includes a thread 104 extending between proximal portion 102and distal tip 105. Beginning thread 104 at distal tip 105 allows shaft103 to engage and anchor into the bone immediately upon contact. As seenin FIGS. 1-3, thread 104 includes along a length thereof a serratedportion 106 that has individual serrations 107. Serrated portion 106extends along about 35 to 40 percent of a length of thread 104. Incertain embodiments, serrated portion 106 extends along 35 percent ofthe length of thread 104. In other embodiments, the range of serratedportion 106 can be about 25-45 percent of a length of thread 104, about20-50 percent of a length of thread 104, or about 10-60 percent of alength of thread 104. This ratio of the serrated portion to the overallthread length allows for a consistent feel during manual insertion,regardless of the screw length. The inclusion of serrated portion 106provides a solution for the problems of the prior art, as describedabove. Serrations 107 reduce insertion torque, thereby improving ease ofinsertion, while not compromising pullout strength. Serrated screws alsoallow for a quicker insertion time. The reduced insertion torque reducesthe chance of bone fracturing and breaching. Additionally, serrations107 allow surgeons to retain tactile feedback with minimized energyexertion resulting in greater accuracy during positioning of fastener100, as compared with manual screw insertion of other non-serrated priorart screws.

Shaft 103 is tapered, such that the tapered portion of shaft 103 isdefined by an angle of between 16 and 20 degrees measured betweenlongitudinal axis 108 of the shaft 103 and an axis intersecting outersurfaces of thread 104 at two or more revolutions of thread 104. Incertain embodiments, the tapered portion of shaft 103 extends alongabout 35 percent of the length of thread 104, which can match the lengthof thread 104 along which serrated portion 106 extends. In otherembodiments, the range of tapered portion can be about 25-45 percent ofa length of thread 104, about 20-50 percent of a length of thread 104,or about 10-60 percent of a length of thread 104. This configuration isdesigned so that once a maximum diameter of the threads is reached, theserrated portion 106 ends so that the threads at the maximum diameter donot continue to cut into the bone. Further cutting into the bone by themaximum diameter threads can weaken the engagement between thelater-inserted, non-serrated threads and the bone, which reduces tactilefeedback to the user. Having the tapered portion of shaft 103 andserrated portion 106 both extend along the same amount of the length ofthread 104 (i.e., about 35 percent) allows some resistance at all timesduring insertion of the screw, which is desirable. Other embodiments inaccordance with the present invention may include a shaft that is nottapered.

In the embodiment of FIGS. 1-6, proximal portion 102 of screw body 101includes a proximally-facing flat top surface 126 and a distally-facing,generally spherical surface 125, which interfaces with the tulip toallow for polyaxial movement between the tulip and screw body 101. Asshown in FIGS. 1, 2, and 4, proximal portion 102 includes a projection130 extending from a central portion of flat top surface 126. Proximalportion 102 further includes nubs 135 extending from flat top surface126 at locations around the periphery thereof and surrounding projection130. Nubs 135 are each smaller in size than projection 130. Although theshape of projections 130 and nubs 135 can vary depending on thecorresponding tulip assembly and the corresponding insertioninstruments, in the illustrated embodiment, projections 130 and nubs 135are each rounded to have generally spherically-shaped proximal ends. Asshown more clearly in FIG. 2, in this embodiment, proximal portion 102includes six nubs 135. The number of nubs 135 can vary in otherembodiments.

Thread 104 can have one or more of many cross-sectional areas, such astrapezoidal, square, triangular, rectangular or any other shape known inthe art. As shown in FIGS. 2 and 3, thread 104 includes sidewalls 110 oneither side that extend from an inner diameter of shaft 103 to an outerdiameter of thread 104. Sidewalls 110 are angled such that sidewalls 110that face one another along the longitudinal axis 108 of shaft 103 forman angle α therebetween of 60 degrees. Angle α can be about 60 degrees,as in the depicted embodiment, while it can range between about 55-65degrees in other embodiments. In still other embodiments, angle α canrange between about 45-75 degrees, and between about 40-80 degrees inother embodiments. Thread 104 is configured such that an angle betweensidewall 110 and longitudinal axis 108 of shaft 103 varies along alength of the shaft 103. That is, the angle between the sidewall ofthread 104 and longitudinal axis 108 varies along the path of thread104. In other embodiments, the angle of sidewall 110 can be constant. Inthe embodiment shown in FIGS. 1-6, sidewalls 110 extend towardlongitudinal axis 108 until they intersect with a concave, helical pathbetween adjacent passes of thread 104, with the helical path containingthe inner diameter of shaft 103. In other embodiments, sidewalls 110 mayextend to the inner diameter of the shaft by intersecting or by thehelical path between adjacent passes of thread 104 being flat instead ofconcave.

As shown in FIG. 2, thread 104 includes serrated portion 106 at a distalend of shaft 103 that continuously transitions into a smooth,non-serrated portion at a proximal end of shaft 103. Serrations 107 aregeometrical cut-outs along serrated portion 106 that allow for easierinsertion of fastener 100 into the pedicle bone by reducing theinsertion torque. This reduction in torque limits surgeon fatigue andreduces the chance of fracturing or breaching of the pedicle bone.

Referring to FIG. 3, thread 104 has a tapered portion that defines atapered angle β measured between longitudinal axis 108 of shaft 103 andan axis 120 intersecting distal tip 105 and an outer surface of thread104 at a proximal end of the tapered portion of thread 104. Angle β is25 degrees in the depicted embodiment. In other embodiments, angle β canbe about 25 degrees, or between about 20 to 30 degrees. In still otherembodiments, angle β can range between about 15-35 degrees, and betweenabout 10-40 degrees in other embodiments.

Referring to FIGS. 3 and 4, serrated portion 106 include peaks 112 andtroughs 115 that alternate along serrated portion 106 to defineserrations 107. Peaks 112 are triangular in shape looking along thelongitudinal axis 108. Serrations 107 further include respectivethicknesses 116 measured parallel to longitudinal axis 108 of shaft 103such that successive thicknesses 116 increase in magnitude along aportion of a length of thread 104 toward distal tip 105. Each thickness116 is measured from the distal end to the proximal end of eachserration 107. In other embodiments, successive thicknesses can decreasein magnitude along a portion of a length of thread 104 toward distal tip105 or can remain constant.

In the embodiment of FIGS. 1-6, each peak 112 is disposed at a radialdistance from longitudinal axis 108 of shaft 103 that is greater than aradial distance from longitudinal axis 108 of shaft 103 to an adjacenttrough 115. Each peak 112 has a thickness measured parallel tolongitudinal axis 108 of shaft 103 that is less than a thicknessmeasured parallel to longitudinal axis 108 of shaft 103 of an adjacenttrough 115. In other words, due to the angle between sidewalls 110 ofthread 104 and/or curvature of surfaces 117 of shaft 103, the thicknessis greater at the troughs 115 than at the adjacent peaks 112.

Serrations 107 include respective widths measured perpendicular tolongitudinal axis 108 of shaft 103, such that successive widths decreasein magnitude along a portion of a length of thread 104 toward the distaltip 105. In other embodiments, successive widths can increase inmagnitude along a portion of a length of thread 104 toward distal tip105 or can remain constant.

The pitch of a serration 107 is the distance between adjacent troughs115 that define the serration 107, that is, from a first trough 115across a peak 112 to an adjacent second trough 115. In the embodimentshown in FIGS. 1-6, the pitch of the respective serrations 107progressively and incrementally increases from distal tip 105 towardproximal portion 102 of shaft 103. Thus, the pitch of a serration 107nearer the distal tip 105 is less than the pitch of a serration 107closer to the proximal portion 102. The number of serrations 107 perrevolution of thread 104 is constant, as shown in FIG. 6 in a view fromdistal tip 105 toward proximal portion 102 of shaft 103. This results inthe small pitch of a serration 107 nearer the distal tip 105 becausemore serrations are fit into a revolution of thread 104 that has agenerally smaller diameter due to its tapered structure. Differentangles of the tapered section of thread 104 provide differently shapedserrations 107. In one embodiment, the angle of each face of serration107 measured from a plane through adjacent troughs 115 is 25 degrees. Inother embodiments, the angle of each face of serration 107 measured froma plane through adjacent troughs 115 is between 20-30 degrees, andbetween about 10-40 degrees in other embodiments.

Other embodiments of fasteners in accordance with the present inventionare shown in FIGS. 7 and 8, respectively. Fastener 200 is shown in FIG.7 with serrations 207 running the entire length of thread 204 alongshaft 203. Fastener 300 is shown in FIG. 8 having a cannulated shaft 303that defines a passage 350 along the length of shaft 303. Passage 350can extend along a full length of fastener 300 so that it is accessibleat both the proximal and distal ends thereof, with the distal openingbeing shown in FIG. 7. The proximal opening corresponds with the centralprojection at the proximal portion of fastener 300.

FIGS. 9A-9I each depict different embodiments having a varying number ofserrations per each thread revolution. In each embodiment, the pitchincrementally increases from the distal tip to the proximal portion. Forexample, in the fastener 400 shown in FIG. 9A, there are seventy-two(72) peaks 412 per revolution of thread 404. The pitch is smaller at thedistal tip 405 than on threads 404 closer to the proximal portion 402due to the tapered nature of the screw to allow for the same number ofpeaks per revolution of threads 404.

FIGS. 9A-9C depict fasteners having peaks of one type. For example, inFIG. 9C, peaks 612 are defined by a linear edge 613 at an abutmentbetween surfaces connecting peak 612 with adjacent troughs. FIGS. 9D-9Idepict fasteners having peaks of two types. For example, in FIG. 9F,certain peaks are defined by a linear edge, while peaks 919 are definedby a flat or planar surface at an abutment between surfaces connectingpeak 919 with adjacent troughs. 13. In some embodiments, successivepeaks along the serrated portion can alternate between the linear andplanar peaks.

In another embodiment, shown in FIG. 9J, the threads include two typesof peaks that alternate and differ in height, which is the distance fromthe longitudinal axis of the screw body to the top of the peak. Thisconfiguration forms a double-V cut and can vary with a first tall peak,adjacent to a first short peak, the first short peak adjacent to asecond tall peak, the pattern (tall-short-tall-short) continuing aroundthe threads. The taller peaks can have the same or different height,though both are preferable greater in magnitude than the heights of theshorter peaks, which can be the same or different.

In another embodiment, shown in FIG. 9K, the peaks and troughs arerounded. In another embodiment, shown in FIG. 9L, the threads are squareshaped or rectangular in cross-section, having two linear edges definingeach peak and two linear edges defining each trough. In anotherembodiment, shown in FIG. 9M, the serrations are scalloped such that thepeaks are curved with the troughs forming linear edges.

In another embodiment, shown in FIGS. 9N and 90, the serrations can behelical in the opposite direction of the threads of a fastener 1700.That is, serrations 1707 can have peaks 1712 that extend along helicalcurves winding around fastener 1700 in a direction opposite to thehelical curve along which thread 1704 extends. In this way, peaks 1712are not aligned with or parallel to a longitudinal axis X of shaft 1703.

In another embodiment, shown in FIG. 9P, a fastener has a cutting flutethat extends in a linear direction along an axis angled with respect tothe longitudinal axis of the shaft. In another embodiment, shown in FIG.9Q, a fastener has a cutting flute that extends along a helical pathfrom a distal tip of the shaft. Fasteners according to the presentembodiments can include single or dual lead threads and can include oneor more cutting flutes. A dual lead provides the fastener with superiorpullout strength.

FIG. 10 depicts an embodiment similar to fastener 100 in which afastener 1100 is shown with a screw body 1101 and a tulip 1109. FIG. 11depicts an embodiment similar to that of FIG. 10, but one in which afastener 2100 is not cannulated. An embodiment depicted in FIG. 12 is afastener 3100 in which a tulip 3109 includes retractor blades 3111 thatact as guides during insertion of a spinal rod and can be detached oncethe spinal rod is anchored to tulip 3109. FIG. 13 depicts an embodimentsimilar to fastener 100 in which a fastener 4100 has a longer shaft anda tulip 4109 is angled on its distal surface.

Experimental tests were run with different configurations of screws inaccordance with the embodiments of the present invention. Each screw hasa diameter of 5.0 mm and a length of 35.0 mm, and is further configuredas follows:

Screw A Double Lead Non-Cannulated Cutting Flute No Serrations Screw BDouble Lead Non-Cannulated Cutting Flute Serrations Screw C Double LeadCannulated No Cutting Flute Serrations Screw D Double LeadNon-Cannulated No Cutting Flute Serrations

Screws A-D were tested to determine mean maximum insertion torque. Asshown in FIG. 14, Screws B-D having serrations in accordance with thepresent invention showed superior performance to Screw A, which does notinclude serrations. The mean maximum insertion torque is less for all ofScrews B-D as compared with Screw A. This evidences the desired resultof lowering the insertion torque for a screw by providing serrations, toimprove the performance of manual insertion.

In a serrated bone screw according to the present invention, theserrated portion can be defined as a function of thread length. Keepingthe length of the serrated portion of the thread proportional to thethread length ensures consistent feel irrespective of screw length. Bycreating a proportional relationship, the end user will have the sameexperience despite the screw length. Calculating the length of theserrated portion can be done using the following formula: (SerrationLength)=(Thread Length) times (X), where X equals a constant. Thisresults in a linear relationship between the length of the serratedportion and the thread length. Thus, kits of screws in accordance withthe present invention can include screws of different overall lengthshaving proportional serrated lengths based on a constant value.

In other embodiments, due to manufacturing constraints, it may bedesirable to have fewer unique serration lengths, but still satisfy theneed for a consistent feel. Accordingly, the serrated portion can bedefined using a bucketed proportional approach. For example, if (ScrewLength)≤(X) then (Serration Length)=(Y), where X is a defined ScrewLength and Y is a defined Serration Length. This results in fewer uniqueserration lengths, but provides the same reduced insertion torque to theend user. For instance, by defining five (5) “buckets” of serrationlengths, you can achieve a (Serration Length)/(Thread Length) proportionin a desired range, for example, 0.25 to 0.45. Kits of screws inaccordance with the present invention can include screws of differentlengths having serrated lengths according to these different “buckets”to provide multiple options for a user.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A fastener configured for spinal applications comprising: a head having a channel adapted to receive a spinal rod; and a shaft extending from the head to a distal tip and having a thread, at least a portion of the thread being serrated, wherein the serrated portion of the thread includes serrations having respective widths measured perpendicular to a longitudinal axis of the shaft, successive widths decreasing in magnitude along a portion of a length of the thread toward the distal tip.
 2. The fastener of claim 1, wherein an angle between a sidewall of the thread and the longitudinal axis of the shaft varies along a length of the shaft.
 3. The fastener of claim 1, wherein the serrations have respective thicknesses measured parallel to the longitudinal axis of the shaft, successive thicknesses increasing in magnitude along a portion of a length of the thread toward the distal tip.
 4. The fastener of claim 1, wherein at least a portion of the head is polyaxially movable with respect to the shaft.
 5. The fastener of claim 1, wherein the shaft is cannulated.
 6. The fastener of claim 1, wherein the shaft is tapered.
 7. The fastener of claim 6, wherein the tapered shaft is defined by an angle of between 16 and 20 degrees measured between the longitudinal axis of the shaft and an axis intersecting outer surfaces of the thread at two or more revolutions thereof.
 8. The fastener of claim 1, wherein sidewalls of the thread that face one another form an angle therebetween of about 55 to 65 degrees.
 9. The fastener of claim 1, wherein the head is monoaxially attached to the shaft.
 10. A fastener configured for spinal applications comprising: a head having a channel adapted to receive a spinal rod; a shaft coupled with the head, the shaft having a distal tip; a thread extending between the head and the distal tip; and a serrated portion extending along at least a portion of the thread, the serrated portion including peaks and troughs, wherein the peaks include a first type of peak defined by a linear edge at an abutment between surfaces connecting the peak with adjacent troughs and a second type of peak different from the first type of peak and defined by a planar surface at an abutment between surfaces connecting the peak with adjacent troughs, and wherein the shaft includes a cutting flute that extends in a linear direction along an axis angled with respect to a longitudinal axis of the shaft.
 11. The fastener of claim 10, wherein each peak is disposed at a radial distance from the longitudinal axis of the shaft that is greater than a radial distance from the longitudinal axis of the shaft to an adjacent trough, each peak having a thickness measured parallel to the longitudinal axis of the shaft that is less than a thickness measured parallel to the longitudinal axis of the shaft of an adjacent trough.
 12. The fastener of claim 10, wherein successive peaks along the serrated portion alternate between the first type of peak and the second type of peak.
 13. The fastener of claim 10, wherein the first type of peak varies in height along a length of the serrated portion and includes a first short peak with a first radius measured from the longitudinal axis of the shaft adjacent to a first tall peak with a second radius, which in turn is adjacent to a second short peak with a third radius, adjacent to a second tall peak with a fourth radius, the first and third radii being similar and both lesser in dimension than the second and fourth radii.
 14. The fastener of claim 10, wherein the serrated portion includes a progressively increasing pitch from the tip toward the head.
 15. The fastener of claim 10, wherein the peaks extend along helical curves winding around the shaft in a direction opposite to a helical curve along which the thread extends.
 16. The fastener of claim 10, wherein the peaks extend along axes that are parallel to or aligned with the longitudinal axis of the shaft.
 17. A fastener configured for spinal applications comprising: a head having a channel adapted to receive a spinal rod; a shaft coupled with the head, the shaft having a distal tip; a thread extending between the head and the distal tip; and a serrated portion extending along at least a portion of the thread, the serrated portion including peaks and troughs, wherein the peaks include a first type of peak defined by a linear edge at an abutment between surfaces connecting the peak with adjacent troughs and a second type of peak different from the first type of peak and defined by a planar surface at an abutment between surfaces connecting the peak with adjacent troughs, and wherein the shaft includes a cutting flute that extends along a helical path from the distal tip of the shaft. 